9.1 The science of genetics has ancient roots - · PDF fileSex Chromosomes and Sex-Linked...

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© 2012 Pearson Education, Inc. Lecture by Edward J. Zalisko

PowerPoint Lectures for

Campbell Biology: Concepts & Connections, Seventh EditionReece, Taylor, Simon, and Dickey

Chapter 9 Patterns of Inheritance

Dogs are one of man’s longest genetic experiments.

– Over thousands of years, humans have chosen and mated dogs with specific traits.

– The result has been an incredibly diverse array of dogs with distinct

– body types and

– behavioral traits.

Introduction

© 2012 Pearson Education, Inc.

Figure 9.0_1Chapter 9: Big Ideas

Mendel’s Laws Variations onMendel’s Laws

The Chromosomal Basisof Inheritance

Sex Chromosomes andSex-Linked Genes

Figure 9.0_2

MENDEL’S LAWS


9.1 The science of genetics has ancient roots

Pangenesis, proposed around 400 BCE by Hippocrates, was an early explanation for inheritance that suggested that

– particles called pangenes came from all parts of the organism to be incorporated into eggs or sperm and

– characteristics acquired during the parents’ lifetime could be transferred to the offspring.

Aristotle rejected pangenesis and argued that instead of particles, the potential to produce the traits was inherited.


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Figure 9.1

9.1 The science of genetics has ancient roots

The idea that hereditary materials mix in forming offspring, called the blending hypothesis, was

– suggested in the 19th century by scientists studying plants but

– later rejected because it did not explain how traits that disappear in one generation can reappear in later generations.


9.2 Experimental genetics began in an abbey garden

Heredity is the transmission of traits from one generation to the next.

Genetics is the scientific study of heredity.

Gregor Mendel

– began the field of genetics in the 1860s,

– deduced the principles of genetics by breeding garden peas, and

– relied upon a background of mathematics, physics, and chemistry.



In 1866, Mendel

– correctly argued that parents pass on to their offspring discrete “heritable factors” and

– stressed that the heritable factors (today called genes), retain their individuality generation after generation.

A heritable feature that varies among individuals, such as flower color, is called a character.

Each variant for a character, such as purple or white flowers, is a trait.


Figure 9.2A


True-breeding varieties result when self-fertilization produces offspring all identical to the parent.

The offspring of two different varieties are hybrids.

The cross-fertilization is a hybridization, or genetic cross.

True-breeding parental plants are the P generation.

Hybrid offspring are the F1 generation.

A cross of F1 plants produces an F2 generation.


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Figure 9.2B

StamenCarpel

Petal

Figure 9.2C_s1

Removal ofstamens

Carpel

White

Stamens

Transferof pollenPurpleParents

(P)

2

1

Figure 9.2C_s2

Removal ofstamens

Carpel

White

Stamens

Transferof pollenPurple

Carpel maturesinto pea pod

Parents(P)

2

3

1

Figure 9.2C_s3

Removal ofstamens

Carpel

White

Stamens

Transferof pollenPurple

Carpel maturesinto pea pod

Seeds frompod planted

Offspring(F1)

Parents(P)

2

3

1

4

Figure 9.2DCharacter Traits

Dominant Recessive

Flower color

Purple White

Flower position

Axial Terminal

Seed colorYellow Green

Seed shapeRound Wrinkled

Pod shape

Inflated Constricted

Pod color

Green Yellow

Stem length

Tall Dwarf

Figure 9.2D_1

Character TraitsDominant Recessive

Flower color

Purple White

Flower position

Axial Terminal

Seed colorYellow Green

Seed shape

Round Wrinkled

4

Figure 9.2D_2

CharacterDominant Recessive

Traits

Pod shape

Inflated Constricted

Pod color

Green Yellow

Stem length

Tall Dwarf

9.3 Mendel’s law of segregation describes the inheritance of a single character

A cross between two individuals differing in a single character is a monohybrid cross.

Mendel performed a monohybrid cross between a plant with purple flowers and a plant with white flowers.

– The F1 generation produced all plants with purple flowers.

– A cross of F1 plants with each other produced an F2generation with ¾ purple and ¼ white flowers.


Figure 9.3A_s1

The Experiment

P generation(true-breedingparents)

Purpleflowers

Whiteflowers

Figure 9.3A_s2

The Experiment


F1 generation

Purpleflowers

Whiteflowers

All plants havepurple flowers

Figure 9.3A_s3

The Experiment


F1 generation

F2 generation

of plantshave purple flowers

of plantshave white flowers

Purpleflowers

Whiteflowers

All plants havepurple flowers

Fertilizationamong F1 plants(F1 F1)

34

14


The all-purple F1 generation did not produce light purple flowers, as predicted by the blending hypothesis.

Mendel needed to explain why

– white color seemed to disappear in the F1 generation and

– white color reappeared in one quarter of the F2 offspring.

Mendel observed the same patterns of inheritance for six other pea plant characters.


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Mendel developed four hypotheses, described below using modern terminology.

1. Alleles are alternative versions of genes that account for variations in inherited characters.

2. For each characteristic, an organism inherits two alleles, one from each parent. The alleles can be the same or different.

– A homozygous genotype has identical alleles.

– A heterozygous genotype has two different alleles.



3. If the alleles of an inherited pair differ, then one determines the organism’s appearance and is called the dominant allele. The other has no noticeable effect on the organism’s appearance and is called the recessive allele.

– The phenotype is the appearance or expression of a trait.

– The genotype is the genetic makeup of a trait.

– The same phenotype may be determined by more than one genotype.



4. A sperm or egg carries only one allele for each inherited character because allele pairs separate (segregate) from each other during the production of gametes. This statement is called the law of segregation.

Mendel’s hypotheses also explain the 3:1 ratio in the F2 generation.

– The F1 hybrids all have a Pp genotype.

– A Punnett square shows the four possible combinations of alleles that could occur when these gametes combine.


Figure 9.3B_s1The Explanation

P generation Genetic makeup (alleles)

Purple flowers White flowers

Gametes All p

ppPP

PAll


P generation

F1 generation(hybrids)

Genetic makeup (alleles)


Gametes All p

ppPP

P pGametes

All Pp

21

21

PAll


P generation

F1 generation(hybrids)

F2 generation

Genetic makeup (alleles)


Gametes P All p

ppPP

P

P

P

p

p

p

PP Pp

Pp pp

Eggsfrom F1

plant

Gametes

Fertilization

All Pp

Allelessegregate

Phenotypic ratio3 purple : 1 white

Genotypic ratio1 PP : 2 Pp : 1 pp

Sperm from F1 plant

21

21

All

6

Figure 9.3B_4

F2 generation

P

P

p

p

PP Pp

Pp pp

Eggsfrom F1

plant

Phenotypic ratio3 purple : 1 white

Genotypic ratio1 PP : 2 Pp : 1 pp

Sperm from F1 plant

9.4 Homologous chromosomes bear the alleles for each character

A locus (plural, loci) is the specific location of a gene along a chromosome.

For a pair of homologous chromosomes, alleles of a gene reside at the same locus.

– Homozygous individuals have the same allele on both homologues.

– Heterozygous individuals have a different allele on each homologue.


Figure 9.4

P

P

a

a

B

b

PP aa Bb

Dominantallele

Recessiveallele

Gene loci

Homologouschromosomes

Genotype:Heterozygous,with one dominantand one recessiveallele

Homozygousfor the recessiveallele

Homozygousfor the dominantallele

9.5 The law of independent assortment is revealed by tracking two characters at once

A dihybrid cross is a mating of parental varieties that differ in two characters.

Mendel performed the following dihybrid cross with the following results:– P generation: round yellow seeds wrinkled green seeds

– F1 generation: all plants with round yellow seeds

– F2 generation:

– 9/16 had round yellow seeds

– 3/16 had wrinkled yellow seeds

– 3/16 had round green seeds

– 1/16 had wrinkled green seeds


Figure 9.5A

41

41

41

41

41

41

41

41

169

163

163

161

21

21

21

21

F1 generation

F2 generation

P generation

Gametes

Sperm

Eggs

Yellowround

Greenround

Yellowwrinkled

Greenwrinkled

RRYY rryy

RY ry

RrYy

The hypothesis of dependent assortmentData did not support; hypothesis refuted

The hypothesis of independent assortmentActual results; hypothesis supported

RY

RY

ry

ry

Eggs

RY

RY

rY

rY

Ry

Ry ry

ry

RRYY RrYY RRYy RrYy

RrYY rrYY RrYy rrYy

RRYy RrYy RRyy Rryy

RrYy rrYy Rryy rryy

Sperm

Figure 9.5A_1

F1 generation

P generation

Gametes

RRYY rryy

RY ry

RrYy

7

Figure 9.5A_2

21

F2 generation

The hypothesis of dependent assortmentData did not support; hypothesis refuted

RY

RY

ry

ry

Eggs

Sperm

21

21

21

F1 generation RrYy

Figure 9.5A_3

41

Sperm

Eggs

Yellowround

Greenround

Yellowwrinkled

Greenwrinkled

The hypothesis of independent assortmentActual results; hypothesis supported

RY

RY

rY

rY

Ry

Ry ry

ry

RRYY RrYY RRYy RrYy

RrYY rrYY RrYy rrYy

RRYy RrYy RRyy Rryy

RrYy rrYy Rryy rryy

41

41

41

41

41

41

41

169

163

161

F1 generation RrYy

163

Figure 9.5B

Phenotypes

Genotypes

Black coat,normal vision

B_N_

Black coat,blind (PRA)

B_nn

Blind

Chocolate coat,normal vision

bbN_

Blind

Blind Blind

Chocolate coat,blind (PRA)

bbnn

Mating of double heterozygotes (black coat, normal vision)BbNn BbNn

Phenotypic ratioof the offspring

9Black coat,

normal vision

3Black coat,blind (PRA)

1Chocolate coat,

blind (PRA)

3Chocolate coat,normal vision

Figure 9.5B_1

Phenotypes

Genotypes

Phenotypes

Genotypes

Blind

Blind

Black coat,normal vision

B_N_

Black coat,blind (PRA)

B_nn

Chocolate coat,normal vision

bbN_

Chocolate coat,blind (PRA)

bbnn

Figure 9.5B_2

Blind Blind

Mating of double heterozygotes (black coat, normal vision)

BbNn BbNn

Phenotypic ratio of the offspring

9Black coat,

normal vision

3Black coat,blind (PRA)

1Chocolate coat,

blind (PRA)

3Chocolate coat,normal vision


Mendel needed to explain why the F2 offspring

– had new nonparental combinations of traits and

– a 9:3:3:1 phenotypic ratio.

Mendel

– suggested that the inheritance of one character has no effect on the inheritance of another,

– suggested that the dihybrid cross is the equivalent to two monohybrid crosses, and

– called this the law of independent assortment.


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The following figure demonstrates the law of independent assortment as it applies to two characters in Labrador retrievers:

– black versus chocolate color,

– normal vision versus progressive retinal atrophy.


9.6 Geneticists can use the testcross to determine unknown genotypes

A testcross is the mating between an individual of unknown genotype and a homozygous recessive individual.

A testcross can show whether the unknown genotype includes a recessive allele.

Mendel used testcrosses to verify that he had true-breeding genotypes.

The following figure demonstrates how a testcross can be performed to determine the genotype of a Lab with normal eyes.


Figure 9.6

What is the genotype of the black dog?

Two possibilities for the black dog:

Testcross

Genotypes

Gametes

Offspring All black 1 black : 1 chocolate

or

B_? bb

BbBB

B B

b b

b

Bb Bb bb

9.7 Mendel’s laws reflect the rules of probability

Using his strong background in mathematics, Mendel knew that the rules of mathematical probability affected

– the segregation of allele pairs during gamete formation and

– the re-forming of pairs at fertilization.

The probability scale ranges from 0 to 1. An event that is

– certain has a probability of 1 and

– certain not to occur has a probability of 0.


9.7 Mendel’s laws reflect the rules of probability

The probability of a specific event is the number of ways that event can occur out of the total possible outcomes.

Determining the probability of two independent events uses the rule of multiplication, in which the probability is the product of the probabilities for each event.

The probability that an event can occur in two or more alternative ways is the sum of the separate probabilities, called the rule of addition.


Figure 9.7

F1 genotypes

Formationof eggs

Formationof sperm

Bb female Bb male

Sperm

F2 genotypes Eggs

B

B B B B

B

b

b

bb

b b

21

21

21

21

21

21

41

41

41

41

( )

9

9.8 CONNECTION: Genetic traits in humans can be tracked through family pedigrees

In a simple dominant-recessive inheritance of dominant allele A and recessive allele a,

– a recessive phenotype always results from a homozygous recessive genotype (aa) but

– a dominant phenotype can result from either

– the homozygous dominant genotype (AA) or

– a heterozygous genotype (Aa).

Wild-type traits, those prevailing in nature, are not necessarily specified by dominant alleles.


Figure 9.8ADominant Traits Recessive Traits

Freckles No freckles

Widow’s peak Straight hairline

Free earlobe Attached earlobe

Figure 9.8A_1

Freckles

Figure 9.8A_2

No freckles

Figure 9.8A_3

Widow’s peak

Figure 9.8A_4

Straight hairline

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Figure 9.8A_5

Free earlobe

Figure 9.8A_6

Attached earlobe

9.8 CONNECTION: Genetic traits in humans can be tracked through family pedigrees

The inheritance of human traits follows Mendel’s laws.

A pedigree

– shows the inheritance of a trait in a family through multiple generations,

– demonstrates dominant or recessive inheritance, and

– can also be used to deduce genotypes of family members.


Figure 9.8B

First generation(grandparents)

Second generation(parents, aunts,and uncles)

Third generation(two sisters)

Female MaleAttachedFree

Ff Ff Ffff

FfFfff ff ff

ff

FForFf

FForFf

9.9 CONNECTION: Many inherited disorders in humans are controlled by a single gene

Inherited human disorders show either

1. recessive inheritance in which

– two recessive alleles are needed to show disease,

– heterozygous parents are carriers of the disease-causing allele, and

– the probability of inheritance increases with inbreeding, mating between close relatives.

2. dominant inheritance in which

– one dominant allele is needed to show disease and

– dominant lethal alleles are usually eliminated from the population.


Figure 9.9A

Parents

Offspring

Sperm

Eggs

NormalDd

NormalDd

D

D

d

d

DDNormal

DdNormal(carrier)

DdNormal(carrier)

ddDeaf

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9.9 CONNECTION: Many inherited disorders in humans are controlled by a single gene

The most common fatal genetic disease in the United States is cystic fibrosis (CF), resulting in excessive thick mucus secretions. The CF allele is

– recessive and

– carried by about 1 in 31 Americans.

Dominant human disorders include

– achondroplasia, resulting in dwarfism, and

– Huntington’s disease, a degenerative disorder of the nervous system.


Table 9.9

Figure 9.9B

New technologies offer ways to obtain genetic information

– before conception,

– during pregnancy, and

– after birth.

Genetic testing can identify potential parents who are heterozygous carriers for certain diseases.

9.10 CONNECTION: New technologies can provide insight into one’s genetic legacy


Several technologies can be used for detecting genetic conditions in a fetus.

– Amniocentesis extracts samples of amniotic fluid containing fetal cells and permits

– karyotyping and

– biochemical tests on cultured fetal cells to detect other conditions, such as Tay-Sachs disease.

– Chorionic villus sampling removes a sample of chorionic villus tissue from the placenta and permits similar karyotyping and biochemical tests.



Video: Ultrasound of Human Fetus

Figure 9.10AAmniocentesis

Ultrasoundtransducer

Fetus

Placenta

UterusCervix

Amniotic fluidextracted

Centrifugation

Amniotic fluid

Fetal cells

Culturedcells

Several hours

Several weeks

Several weeks

Biochemicaland geneticstests

Several hours

Several hours

Fetal cells

Cervix

Uterus

Chorionicvilli

PlacentaFetus

Ultrasoundtransducer

Tissue extractedfrom the chorionic villi

Chorionic Villus Sampling (CVS)

Karyotyping

12

Blood tests on the mother at 14–20 weeks of pregnancy can help identify fetuses at risk for certain birth defects.

Fetal imaging enables a physician to examine a fetus directly for anatomical deformities. The most common procedure is ultrasound imaging, using sound waves to produce a picture of the fetus.

Newborn screening can detect diseases that can be prevented by special care and precautions.



Figure 9.10B

Figure 9.10B_1 Figure 9.10B_2

New technologies raise ethical considerations that include

– the confidentiality and potential use of results of genetic testing,

– time and financial costs, and

– determining what, if anything, should be done as a result of the testing.



VARIATIONS ON MENDEL’S LAWS


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9.11 Incomplete dominance results in intermediate phenotypes

Mendel’s pea crosses always looked like one of the parental varieties, called complete dominance.

For some characters, the appearance of F1 hybrids falls between the phenotypes of the two parental varieties. This is called incomplete dominance, in which

– neither allele is dominant over the other and

– expression of both alleles occurs.


Figure 9.11AP generation

F1 generation

F2 generation

21

21

21

21

21

21

Gametes

Gametes

Eggs

Sperm

RedRR

Whiterr

Pink hybridRr

R

R

R

R

r

r

r

r

RR rR

Rr rr

Figure 9.11A_1

P generation

Gametes

RedRR

Whiterr

R r

Figure 9.11A_2

F1 generation

21

Gametes

Pink hybridRr

R r21

21

21

21

21

Figure 9.11A_3

F2 generation

Eggs

Sperm

R

R

r

r

RR rR

Rr rr

9.11 Incomplete dominance results in intermediate phenotypes

Incomplete dominance does not support the blending hypothesis because the original parental phenotypes reappear in the F2 generation.

One example of incomplete dominance in humans is hypercholesterolemia, in which

– dangerously high levels of cholesterol occur in the blood and

– heterozygotes have intermediately high cholesterol levels.


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Figure 9.11B

Normal Mild disease Severe disease

Phenotypes

Cell

LDLreceptor

LDL

HHHomozygous

for ability to makeLDL receptors

hhHomozygous

for inability to makeLDL receptors

GenotypesHh

Heterozygous

9.12 Many genes have more than two alleles in the population

Although an individual can at most carry two different alleles for a particular gene, more than two alleles often exist in the wider population.

Human ABO blood group phenotypes involve three alleles for a single gene.

The four human blood groups, A, B, AB, and O, result from combinations of these three alleles.

The A and B alleles are both expressed in heterozygous individuals, a condition known as codominance.


9.12 Many genes have more than two alleles in the population

In codominance,

– neither allele is dominant over the other and

– expression of both alleles is observed as a distinct phenotype in the heterozygous individual.

– AB blood type is an example of codominance.


Figure 9.12

Blood Group(Phenotype) Genotypes

Carbohydrates Presenton Red Blood Cells

AntibodiesPresentin Blood

A

B

AB

O

IAIA

orIAi

IBIB

orIBi

IAIB

ii

Carbohydrate A

Carbohydrate B

Carbohydrate A

and

Carbohydrate B

Neither

Anti-B

Anti-A

Anti-B

Anti-A

None

No reaction Clumping reaction

O A B AB

Reaction When Blood from Groups Below Is Mixedwith Antibodies from Groups at Left

Figure 9.12_1

Blood Group(Phenotype) Genotypes

Carbohydrates Presenton Red Blood Cells

A

B

AB

O

IAIA

orIAi

IBIB

orIBi

IAIB

ii

Carbohydrate A

Carbohydrate B

Carbohydrate A

and

Carbohydrate B

Neither

Figure 9.12_2

AntibodiesPresentin Blood

Anti-B

Anti-A

Anti-B

Anti-A

None

O A B AB

Reaction When Blood from Groups Below Is Mixed with Antibodies from Groups at Left

Blood Group(Phenotype)

A

B

AB

O

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9.13 A single gene may affect many phenotypic characters

Pleiotropy occurs when one gene influences many characteristics.

Sickle-cell disease is a human example of pleiotropy. This disease

– affects the type of hemoglobin produced and the shape of red blood cells and

– causes anemia and organ damage.

– Sickle-cell and nonsickle alleles are codominant.

– Carriers of sickle-cell disease are resistant to malaria.


Figure 9.13A

Figure 9.13BAn individual homozygous for the sickle-cell allele

Produces sickle-cell (abnormal) hemoglobin

The abnormal hemoglobin crystallizes,causing red blood cells to become sickle-shaped

Sickled cell

The multiple effects of sickled cells

Damage to organs Other effects

Kidney failureHeart failureSpleen damageBrain damage (impaired

mental function,paralysis)

Pain and feverJoint problemsPhysical weaknessAnemiaPneumonia and other

infections

9.14 A single character may be influenced by many genes

Many characteristics result from polygenic inheritance, in which a single phenotypic character results from the additive effects of two or more genes.

Human skin color is an example of polygenic inheritance.


Figure 9.14P generation

F1 generation

F2 generation

Eggs

Sperm

Skin color

Fra

ctio

n o

f p

op

ula

tio

n

aabbcc(very light)

AABBCC(very dark)

AaBbCc AaBbCc

81

6415

6420

646

641

6415

646

641

81

81

81

81

81

81

81

81

81

81

81

81

81

81

81

Figure 9.14_1

P generation

F1 generation

aabbcc(very light)

AABBCC(very dark)

AaBbCc AaBbCc

16

Figure 9.14_2

F2 generation

Eggs

Sperm

81

81

81

81

81

81

81

81

81

81

81

81

81

81

81

81

641

646

6415

6420

641

646

6415

Figure 9.14_3

Skin color

Fra

ctio

n o

f p

op

ula

tio

n

6420

6415

646

641

9.15 The environment affects many characters

Many characters result from a combination of heredity and the environment. For example,

– skin color is affected by exposure to sunlight,

– susceptibility to diseases, such as cancer, has hereditary and environmental components, and

– identical twins show some differences.

Only genetic influences are inherited.


Figure 9.15A

Figure 9.15B

THE CHROMOSOMAL BASIS OF INHERITANCE


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9.16 Chromosome behavior accounts for Mendel’s laws

The chromosome theory of inheritance states that

– genes occupy specific loci (positions) on chromosomes and

– chromosomes undergo segregation and independent assortment during meiosis.


9.16 Chromosome behavior accounts for Mendel’s laws

Mendel’s laws correlate with chromosome separation in meiosis.

– The law of segregation depends on separation of homologous chromosomes in anaphase I.

– The law of independent assortment depends on alternative orientations of chromosomes in metaphase I.


Figure 9.16_s1

F1 generation All yellow round seeds(RrYy)

Meta-phase I

of meiosis

Y y

R r r

r

R

R

Y

Y

y

y

Figure 9.16_s2

F1 generation All yellow round seeds(RrYy)

Meta-phase I

of meiosis

Anaphase I

Metaphase IIR

y

r

YY

R r

y

Y

Y

y

y

R

R

r

r r

r

r

R

R

R

Y

Y

Y

y

y

y

Figure 9.16_s3

F1 generation

41

41

41

41

All yellow round seeds(RrYy)

Meta-phase I

of meiosis

Anaphase I

Metaphase II

Fertilization

Gametes

F2 generation 9 :3 :3 :1

RY ry rY Ry

R

R

R

yy

y

rr

r

YY

Y

YY

Y

R

R R

r

rr

y

yy

Y

Y

y

y

R

R

r

r r

r

r

R

R

R

Y

Y

Y

y

y

y

Figure 9.16_4

Sperm

Eggs

Yellowround

Greenround

Yellowwrinkled

Greenwrinkled

RY

RY

rY

rY

Ry

Ry ry

ry

RRYY RrYY RRYy RrYy

RrYY rrYY RrYy rrYy

RRYy RrYy RRyy Rryy

RrYy rrYy Rryy rryy

41

169

163

163

161

41

41

41

41

41

41

41

18

9.17 SCIENTIFIC DISCOVERY: Genes on the same chromosome tend to be inherited together

Bateson and Punnett studied plants that did not show a 9:3:3:1 ratio in the F2 generation. What they found was an example of linked genes, which

– are located close together on the same chromosome and

– tend to be inherited together.


Figure 9.17The Experiment

Purple flower

Long pollenPpLl PpLl

Phenotypes Observedoffspring

Prediction(9:3:3:1)

Purple longPurple roundRed longRed round

284212155

215717124

The Explanation: Linked Genes

Parentaldiploid cellPpLl

Meiosis

P L

P L

p l

p lMostgametes

Fertilization

Sperm

Mostoffspring Eggs

3 purple long : 1 red roundNot accounted for: purple round and red long

P L P L

P L

P L

PL

PLp l

p l

p l

p l

pl

pl

Figure 9.17_1

The Experiment

Purple flower

Long pollenPpLl PpLl

Phenotypes Observedoffspring

Prediction(9:3:3:1)

Purple longPurple roundRed longRed round

284212155

215717124

Figure 9.17_2The Explanation: Linked Genes

Parentaldiploid cellPpLl

Meiosis

P L

P L

p l

p lMostgametes

Fertilization

Sperm

Mostoffspring Eggs

3 purple long : 1 red roundNot accounted for: purple round and red long

P L P L

P L

P L

PL

PLp l

p l

p l

p l

pl

pl

9.18 SCIENTIFIC DISCOVERY: Crossing over produces new combinations of alleles

Crossing over between homologous chromosomes produces new combinations of alleles in gametes.

Linked alleles can be separated by crossing over, forming recombinant gametes.

The percentage of recombinants is the recombination frequency.


Figure 9.18A

P L

Tetrad(pair of

homologouschromosomes)

p l

p lp L

p L P lCrossing over

Parental gametes

Recombinant gametes

19

Figure 9.18B Figure 9.18C

The Experiment

Female Male

ggllGgLl

Black body,vestigial wings

Gray body,long wings(wild type)

Offspring

Gray long Black vestigial Gray vestigial Black long

Recombinantphenotypes

Parentalphenotypes

Recombination frequency 0.17 or 17% 391 recombinants2,300 total offspring

965 944 206 185

Offspring

Parental Recombinant

Eggs Sperm

Crossing over

G L

g l g l

g l

g l g l

G l g L

G L g l G l g L g l

g l g l

g l

The Explanation

GgLlFemale

ggllMale

G L

Figure 9.18C_1

The Experiment

Female Male

ggllGgLl

Black body,vestigial wings

Gray body,long wings(wild type)

Offspring: Gray long Black vestigial Gray vestigial Black long

Recombinantphenotypes

Parentalphenotypes

Recombination frequency 0.17 or 17% 391 recombinants

2,300 total offspring

965 944 206 185

Figure 9.18C_2

Offspring

Parental Recombinant

Eggs Sperm

Crossing over

G L

g l g l

g l

g l g l

G l g L

G L g l G l g L g l

g l g l

g l

The Explanation

GgLlFemale

ggllMale

G L

9.19 Geneticists use crossover data to map genes

When examining recombinant frequency, Morgan and his students found that the greater the distance between two genes on a chromosome, the more points there are between them where crossing over can occur.

Recombination frequencies can thus be used to map the relative position of genes on chromosomes.


Figure 9.19A

Section of chromosome carrying linked genes

Recombinationfrequencies

17%

9% 9.5%

g c l

20

Figure 9.19B

Mutant phenotypes

Shortaristae

Blackbody(g)

Cinnabareyes(c)

Vestigialwings(l)

Brown eyes

Red eyes

Normalwings(L)

Redeyes(C)

Graybody(G)

Long aristae(appendageson head)

Wild-type phenotypes

SEX CHROMOSOMES AND SEX-LINKED GENES


9.20 Chromosomes determine sex in many species

Many animals have a pair of sex chromosomes,

– designated X and Y,

– that determine an individual’s sex.

In mammals,

– males have XY sex chromosomes,

– females have XX sex chromosomes,

– the Y chromosome has genes for the development of testes, and

– an absence of the Y allows ovaries to develop.


Figure 9.20A

X

Y

Figure 9.20B

Parents(diploid)

Gametes(haploid)

Offspring(diploid)

Female

Female

Male

Male

EggSperm

44

XY

44

XX

22X

22Y

22X

44

XX

44

XY

Figure 9.20B_1

21


Grasshoppers, roaches, and some other insects have an X-O system, in which

– O stands for the absence of a sex chromosome,

– females are XX, and

– males are XO.

In certain fishes, butterflies, and birds,

– the sex chromosomes are Z and W,

– males are ZZ, and

– females are ZW.


Figure 9.20C

22

XX

22X

Male Female

Figure 9.20C_1 Figure 9.20D

76

ZW

76

ZZ

Male Female

Figure 9.20D_1


Some organisms lack sex chromosomes altogether.

In bees, sex is determined by chromosome number.

– Females are diploid.

– Males are haploid.


22

Figure 9.20E

Male Female

16 32

Figure 9.20E_1


In some animals, environmental temperature determines the sex.

– For some species of reptiles, the temperature at which the eggs are incubated during a specific period of development determines whether the embryo will develop into a male or female.

– Global climate change may therefore impact the sex ratio of such species.


Sex-linked genes are located on either of the sex chromosomes.

The X chromosome carries many genes unrelated to sex.

The inheritance of white eye color in the fruit fly illustrates an X-linked recessive trait.

9.21 Sex-linked genes exhibit a unique pattern of inheritance


Figure 9.21A Figure 9.21A_1

23

Figure 9.21A_2 Figure 9.21BMaleFemale

Sperm

Eggs

R red-eye alleler white-eye allele

XR

Xr

XrYXRXR

XRXr XRY

Y

Figure 9.21C


MaleFemale

Sperm

Eggs

XR

xR

XRYXRXr

Y

Xr XrY

XRYXRXR

XrXR

Figure 9.21D


MaleFemale

Sperm

Eggs

XrY

XRXr

XrXrXr

XR

Xr Y

XrY

XRY

XRXr

9.22 CONNECTION: Human sex-linked disorders affect mostly males

Most sex-linked human disorders are

– due to recessive alleles and

– seen mostly in males.

A male receiving a single X-linked recessive allele from his mother will have the disorder.

A female must receive the allele from both parents to be affected.


9.22 CONNECTION: Human sex-linked disorders affect mostly males

Recessive and sex-linked human disorders include

– hemophilia, characterized by excessive bleeding because hemophiliacs lack one or more of the proteins required for blood clotting,

– red-green color blindness, a malfunction of light-sensitive cells in the eyes, and

– Duchenne muscular dystrophy, a condition characterized by a progressive weakening of the muscles and loss of coordination.


24

Figure 9.22

Female MaleHemophilia

Carrier

NormalAlexis

Alexandra CzarNicholas IIof Russia

QueenVictoria

Alice Louis

Albert

Figure 9.22_1

9.23 EVOLUTION CONNECTION: The Y chromosome provides clues about human male evolution

The Y chromosome provides clues about human male evolution because

– Y chromosomes are passed intact from father to son and

– mutations in Y chromosomes can reveal data about recent shared ancestry.


Figure 9.23

1. Describe pangenesis theory and the blending hypothesis. Explain why both ideas are now rejected.

2. Define and distinguish between true-breeding organisms, hybrids, the P generation, the F1

generation, and the F2 generation.

3. Define and distinguish between the following pairs of terms: homozygous and heterozygous; dominant allele and recessive allele; genotype and phenotype. Also, define a monohybrid cross and a Punnett square.

You should now be able to


4. Explain how Mendel’s law of segregation describes the inheritance of a single characteristic.

5. Describe the genetic relationships between homologous chromosomes.

6. Explain how Mendel’s law of independent assortment applies to a dihybrid cross.

7. Explain how and when the rule of multiplication and the rule of addition can be used to determine the probability of an event.



25

8. Explain how family pedigrees can help determine the inheritance of many human traits.

9. Explain how recessive and dominant disorders are inherited. Provide examples of each.

10. Compare the health risks, advantages, and disadvantages of the following forms of fetal testing: amniocentesis, chorionic villus sampling, and ultrasound imaging.



11. Describe the inheritance patterns of incomplete dominance, multiple alleles, codominance, pleiotropy, and polygenic inheritance.

12. Explain how the sickle-cell allele can be adaptive.

13. Explain why human skin coloration is not sufficiently explained by polygenic inheritance.

14. Define the chromosome theory of inheritance. Explain the chromosomal basis of the laws of segregation and independent assortment.



15. Explain how linked genes are inherited differently from nonlinked genes.

16. Describe T. H. Morgan’s studies of crossing over in fruit flies. Explain how Sturtevant created linkage maps.

17. Explain how sex is genetically determined in humans and the significance of the SRY gene.

18. Describe patterns of sex-linked inheritance and examples of sex-linked disorders.

19. Explain how the Y chromosome can be used to trace human ancestry.



Figure 9.UN01

Fertilization

Meiosis

Homologouschromosomes

Alleles, residingat the same locus

Paired alleles,different formsof a gene Haploid gametes

(allele pairs separated)

Gametefrom theother parent

Diploid zygote(containingpaired alleles)

Figure 9.UN02

Incompletedominance

RedRR

Whiterr

PinkRr

Figure 9.UN03

Singlegene

PleiotropyMultiple characters

26

Figure 9.UN04

Multiplegenes

Polygenicinheritance

Single characters(such as skin color)

Figure 9.UN05

chromosomes

Genes

heterozygous

locatedon

alternativeversions called

at specificlocations called

if both are the same,the genotype is called

if different, thegenotype is called

the expressedallele is called

the unexpressedallele is called

inheritance when the phenotypeis in between is called

(a)

(b) (c)

(d) (e)

(f)

Figure 9.UN06

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